Flame retardant EMI shielding gasket

ABSTRACT

A flame retardant, electromagnetic interference (EMI) shielding gasket construction. The construction includes a resilient core member formed of a foamed elastomeric material, an electrically-conductive fabric member surrounding the outer surface of the core member, and a flame retardant layer coating at least a portion of the interior surface of the fabric member. The flame retardant layer is effective to afford the gasket construction with a flame class rating of V-0 under Underwriter&#39;s Laboratories (UL) Standard No. 94.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of U.S. application Ser. No.10/142,803, now U.S. Pat. No. ______, which is a continuation of U.S.application Ser. No. 09/883,785, filed Jun. 18, 2001, now U.S. Pat. No.6,387,523; which is a continuation of U.S. application Ser. No.09/250,338, filed Feb. 16, 1999, now U.S. Pat. No. 6,428,393 andclaiming priority to U.S. provisional application Serial No. 60/076,370,filed Feb. 27, 1998, the disclosure of each of which is expresslyincorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates broadly to electrically-conductive,flame retardant materials for use in electromagnetic interference (EMI)shielding, and to a method of manufacturing the same, and moreparticularly to an electrically-conductive fabric having a layer of aflame retardant coating applied to one surface thereof for use as asheathing within an EMI shielding gasket.

[0003] The operation of electronic devices including televisions,radios, computers, medical instruments, business machines,communications equipment, and the like is attended by the generation ofelectromagnetic radiation within the electronic circuitry of theequipment. Such radiation often develops as a field or as transientswithin the radio frequency band of the electromagnetic spectrum, i.e.,between about 10 KHz and 10 GHz, and is termed “electromagneticinterference” or “EMI” as being known to interfere with the operation ofother proximate electronic devices.

[0004] To attenuate EMI effects, shielding having the capability ofabsorbing and/or reflecting EMI energy may be employed both to confinethe EMI energy within a source device, and to insulate that device orother “target” devices from other source devices. Such shielding isprovided as a barrier which is inserted between the source and the otherdevices, and typically is configured as an electrically conductive andgrounded housing which encloses the device. As the circuitry of thedevice generally must remain accessible for servicing or the like, mosthousings are provided with openable or removable accesses such as doors,hatches, panels, or covers. Between even the flattest of these accessesand its corresponding mating or faying surface, however, there may bepresent gaps which reduce the efficiency of the shielding by presentingopenings through which radiant energy may leak or otherwise pass into orout of the device. Moreover, such gaps represent discontinuities in thesurface and ground conductivity of the housing or other shielding, andmay even generate a secondary source of EMI radiation by functioning asa form of slot antenna. In this regard, bulk or surface currents inducedwithin the housing develop voltage gradients across any interface gapsin the shielding, which gaps thereby function as antennas which radiateEMI noise. In general, the amplitude of the noise is proportional to thegap length, with the width of the gap having a less appreciable effect.

[0005] For filling gaps within mating surfaces of housings and other EMIshielding structures, gaskets and other seals have been proposed bothfor maintaining electrical continuity across the structure, and forexcluding from the interior of the device such contaminates as moistureand dust. Such seals are bonded or mechanically attached to, orpress-fit into, one of the mating surfaces, and function to close anyinterface gaps to establish a continuous conductive path thereacross byconforming under an applied pressure to irregularities between thesurfaces. Accordingly, seals intended for EMI shielding applications arespecified to be of a construction which not only provides electricalsurface conductivity even while under compression, but which also has aresiliency allowing the seals to conform to the size of the gap. Theseals additionally must be wear resistant, economical to manufacture,and capability of withstanding repeated compression and relaxationcycles. For further information on specifications for EMI shieldinggaskets, reference may be had to Severinsen, J., “Gaskets That BlockEMI,” Machine Design, Vol. 47, No. 19, pp. 74-77 (Aug. 7, 1975).

[0006] Requirements for typical EMI shielding applications often dictatea low impedance, low profile gasket which is deflectable under normalclosure force loads. Other requirements include low cost and a designwhich provides an EMI shielding effectiveness for both the properoperation of the device and compliance, in the United States, withcommercial Federal Communication Commission (FCC) EMC regulations.

[0007] A particularly economical gasket construction, which alsorequires very low closure forces, i.e. less than about 1 lb/inch (0.175N/mm), is marketed by the Chomerics Division of Parker-Hannifin Corp.,Woburn, Mass. under the tradename “Soft-Shield® 5000 Series.” Suchconstruction consists of an electrically-conductive jacket or sheathingwhich is “cigarette” wrapped lengthwise over a polyurethane or otherfoam core. As is described further in U.S. Pat. No. 4,871,477,polyurethane foams generally are produced by the reaction ofpolyisocyanate and a hydroxyl-functional polyol in the presence of ablowing agent. The blowing agent effects the expansion of the polymerstructure into a multiplicity of open or closed cells.

[0008] The jacket is provided as a highly conductive, i.e., about 1Ω-sq., nickel-plated-silver, woven rip-stop nylon which isself-terminating when cut. Advantageously, the jacket may be bonded tothe core in a continuous molding process wherein the foam is blown orexpanded within the jacket as the jacket is wrapped around the expandingfoam and the foam and jacket are passed through a die and into atraveling molding. Similar gasket constructions are shown incommonly-assigned U.S. Pat. No. 5,028,739 and in U.S. Pat. Nos.4,857,668; 5,054,635; 5,105,056; and 5,202,536.

[0009] Many electronic devices, including PC's and communicationequipment, must not only comply with certain FCC requirements, but alsomust meet be approved under certain Underwriter's Laboratories (UL)standards for flame retardancy. In this regard, if each of theindividual components within an electronic device is UL approved, thenthe device itself does not require separate approval. Ensuring ULapproval for each component therefore reduces the cost of compliance forthe manufacturer, and ultimately may result in cheaper goods for theconsumer. For EMI shielding gaskets, however, such gaskets must be madeflame retardant, i.e., achieving a rating of V-0 under UL Std. No. 94,“Tests for Flammability of Plastic Materials for Parts in Devices andAppliances” (1991), without compromising the electrical conductivitynecessary for meeting EMI shielding requirements.

[0010] In this regard, and particularly with respect to EMI shieldinggaskets of the above-described fabric over foam variety, it has longbeen recognized that foamed polymeric materials are flammable and, incertain circumstances, may present a fire hazard. Owing to theircellular structure, high organic content, and surface area, most foammaterials are subject to relatively rapid decomposition upon exposure tofire or high temperatures.

[0011] One approach for imparting flame retardancy to fabric over foamgaskets has been to employ the sheathing as a flame resistant protectivelayer for the foam. Indeed, V-0 rating compliance purportedly has beenachieved by sheathing the foam within an electrically-conductiveNi/Cu-plated fabric to which a thermoplastic sheet is hot nipped orotherwise fusion bonding to the underside thereof. Such fabrics, whichmay be further described in one or more of U.S. Pat. Nos. 4,489,126;4,531,994; 4,608,104; and/or 4,621,013, have been marketed by MonsantoCo., St. Louis, under the tradename “Flectron® Ni/Cu Polyester TaffetaV0.”

[0012] Other fabric over foam gaskets, as is detailed in U.S. Pat. No.4,857,668, incorporate a supplemental layer or coating applied to theinterior surface of the sheath. Such coating may be a flame-retardanturethane formulation which also promotes the adhesion of the sheath tothe foam. The coating additionally may function to reduce bleeding ofthe foam through the fabric which otherwise could compromise theelectrical conductivity of the sheath.

[0013] In view of the foregoing, it will be appreciated that furtherimprovements in the design of flame retardant, fabric-over foam EMIshielding gaskets, as well as sheathing materials therefore, would bewell-received by the electronics industry. Especially desired would be aflame retardant gasket construction which achieves a UL94 rating of V-0.

BROAD STATEMENT OF THE INVENTION

[0014] The present invention is directed to an electrically-conductive,flame retardant material for use in fabric-over-foam EMI shieldinggaskets, and to a method of manufacturing the same. In having a layer ofa flame retardant coating applied to one side of anelectrically-conductive, generally porous fabric, the material of theinvention affords UL94 V-0 protection when used as a jacketing in afabric-over-foam gasket construction. Advantageously, as the flameretardant layer may be wet coated on the fabric without appreciablebleed through, a relatively thin, i.e., 2-4 mil (0.05-0.10 mm), coatinglayer may be provided on one fabric side without compromising theelectrical surface conductivity of the other side. Such a thin coatinglayer, while being sufficient to provide UL94 V-0 protection,nonetheless maintains the drapability the fabric and thereby facilitatesthe construction UL94 V-0 compliant gaskets having complex profiles ornarrow cross-sections down to about 1 mm.

[0015] In a preferred embodiment, the electrically-conductive, flameretardant EMI shielding material of the invention includes a nickel orsilver-plated, woven nylon, polyester, or like fabric on one side ofwhich is wet coated a layer of a flame retardant, acrylic latex emulsionor other fluent resin composition. In accordance with the precepts ofthe method of the invention, the viscosity and hydrodynamic pressure ofthe emulsion are controlled such that the coating does not penetrate orotherwise “bleed through” the uncoated side of the fabric. The surfaceconductivity of the opposite side of the fabric therefore is notcompromised in EMI shielding applications.

[0016] The material of the invention may be employed as a jacket infabric-over-foam EMI shielding gasket constructions, and is particularlyadapted for use in the continuous molding process for such gaskets. Asused within such process, the fabric may be wrapped around the foam as ajacket with coated side thereof being disposed as an interior surfaceadjacent the foam, and the uncoated side being disposed as anelectrically-conductive exterior surface. Advantageously, the coating onthe interior surface of the jacket blocks the pores of the fabric toretain the foam therein without penetrate or bleed through to theexterior surface. In being formed of a acrylic material, the coatedinterior surface of the jacket may function, moreover, depending uponthe composition of the foam, as a compatibilizing or “tie” interlayerwhich promotes the bonding of the foam to the fabric.

[0017] The present invention, accordingly, comprises material and methodpossessing the construction, combination of elements, and arrangement ofparts and steps which are exemplified in the detailed disclosure tofollow. Advantages of the present invention include a flame retardantyet drapable EMI shielding fabric. Additional advantages include aneconomical, flame retardant EMI shielding fabric construction wherein arelatively thin layer of a flame retardant coating may be wet coatedonto one side of an electrically-conductive, woven or other generallyporous EMI shielding fabric without compromising the conductivity of theother side of the fabric. These and other advantages will be readilyapparent to those skilled in the art based upon the disclosure containedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] For a fuller understanding of the nature and objects of theinvention, reference should be had to the following detailed descriptiontaken in connection with the accompanying drawings wherein:

[0019]FIG. 1 is a perspective view of one embodiment of an EMI shieldingmaterial according to the present invention which material includes agenerally planar fabric member on one side of which is coated a layer ofa flame retardant composition, the view being shown with portions beingbroken away to better reveal the structure of the material;

[0020]FIG. 2 is an enlarged cross-sectional view of the EMI shieldingmaterial of FIG. 1 taken through plane represented by line 2-2 of FIG.1;

[0021]FIG. 3 is a top view of the material of FIG. 1 which is magnifiedto reveal the structure of the fabric member thereof;

[0022]FIG. 4 is a perspective cross-sectional view of a length of arepresentative EMI shielding gasket construction according to thepresent invention including a jacket which is formed of the EMIshielding material of FIG. 1;

[0023]FIG. 5 is an end view of the gasket of FIG. 4 which is magnifiedto reveal the structure thereof, and

[0024]FIG. 6 is a schematic, partially cross-sectional view of anillustrative gravity-fed, knife over roll coater as adapted for use inthe manufacture of the EMI shielding material of FIG. 1.

[0025] The drawings will be described further in connection with thefollowing Detailed Description of the Invention.

DETAILED DESCRIPTION OF THE INVENTION

[0026] Certain terminology may be employed in the description to followfor convenience rather than for any limiting purpose. For example, theterms “upper” and “lower” designate directions in the drawings to whichreference is made, with the terms “inner” or “interior” and “outer” or“exterior” referring, respectively, to directions toward and away fromthe center of the referenced element, and the terms “radial” and “axial”referring, respectively, to directions perpendicular and parallel to thelongitudinal central axis of the referenced element. Terminology ofsimilar import other than the words specifically mentioned abovelikewise is to be considered as being used for purposes of conveniencerather than in any limiting sense.

[0027] For the illustrative purposes of the discourse to follow, theelectromagnetic interference (EMI) shielding material herein involved isdescribed in connection with its use as a flame retardant,electrically-conductive jacket for a foam core, EMI shielding gasket asmay be adapted to be received within an interface, such as between adoor, panel, hatch, cover, or other parting line of an electromagneticinterference (EMI) shielding structure. The EMI shielding structure maybe the conductive housing of a computer, communications equipment, orother electronic device or equipment which generates EMI radiation or issusceptible to the effects thereof. The gasket may be bonded or fastenedto, or press-fit into one of a pair of mating surfaces which define theinterface within the housing, and functions between the mating surfacesto seal any interface gaps or other irregularities. That is, while underan applied pressure, the gasket resiliently conforms to any suchirregularities both to establish a continuous conductive path across theinterface, and to environmentally seal the interior of the housingagainst the ingress of dust, moisture, or other contaminates. It will beappreciated, however, that aspects of the present invention may findutility in other EMI shielding applications. Use within those such otherapplications therefore should be considered to be expressly within thescope of the present invention.

[0028] Referring then to the figures, wherein corresponding referencecharacters are used to designate corresponding elements throughout theseveral views, a flame retardant EMI shielding material according to thepresent invention is shown generally at 10 in FIG. 1 as generallyadapted for use as a jacket within for a foam core gasket construction.For purposes of illustration, material sheet 10 is shown to be ofindefinite dimensions which may be cut to size for the particularapplication envisioned. In basic construction, material 10 includes anupper, generally planar and porous fabric member, 12, and a lower, flameretardant coating member, 14.

[0029] Fabric member has at least an electrically-conductive first side,16, and a conductive or non-conductive second side, 18, defining athickness dimension, referenced at “t,” in the cross-sectional view ofFIG. 2, which may vary from about 2-4 mils (0.05-0.10 mm). By“electrically-conductive,” it is meant that the fabric may be renderedconductive, i.e., to a surface resistivity of about 0.1 Ω/sq. or less,by reason of its being constructed of electrically-conductive wire,monofilaments, yarns or other fibers or, alternatively, by reason of atreatment such as a plating or sputtering being applied tonon-conductive fibers to provide an electrically-conductive layerthereon. Preferred electrically-conductive fibers include Monelnickel-copper alloy, silver-plated copper, nickel-clad copper, Ferrex®tin-plated copper-clad steel, aluminum, tin-clad copper, phosphorbronze, carbon, graphite, and conductive polymers. Preferrednon-conductive fibers include cotton, wool, silk, cellulose, polyester,polyamide, nylon, and polyimide monofilaments or yarns which arerendered electrically conductive with a metal plating of copper, nickel,silver, nickel-plated-silver, aluminum, tin, or an alloy thereof. As isknown, the metal plating may applied to individual fiber strands or tothe surfaces of the fabric after weaving, knitting, or otherfabrication.

[0030] While fabrics such as wire meshes, knits, and non-woven clothsand webs may find application, a preferred fabric construction formember 12 is a plain weave nylon or polyester cloth which is madeelectrically conductive with between about 20-40% by weight based on thetotal fabric weight, i.e., 0.01-0.10 g/in², of a silver, nickel-silver,or silver-nickel over copper plating. As may be seen in the magnifiedview of FIG. 1 referenced at 20 in FIG. 3, such cloth is permeable inhaving a plain, generally square weave pattern with pores or openings,one of which is referenced at 22, being defined between the fibers whichare represented schematically at 24. Fibers 24 may be yarns,monofilaments or, preferably, bundles of from about 10-20 filaments orthreads, each having a diameter of between about 10-50 μm. For example,with fibers 24 each being a bundle of such threads with a thread countof between about 1000-3000 per inch and a weave count of between about1000-1500 per inch, 1000-2000 openings per inch will be defined with amean average pore size of between about 0.5-2 mils (12.5-50 μm).

[0031] Although a plain, square weave pattern such as a taffeta, tabby,or ripstop is considered preferred, other weaves such as satins, twills,and the like also should be considered within the scope of the inventionherein involved. A particularly preferred cloth for fabric member 12 isa 4 mil (0.10 mm) thick, 1.8 oz/yd² weight, silver-plated, woven nylonwhich is marketed commercially under the designation “31EN RIPSTOP” bySwift Textile Metalizing Corp., Bloomfield, Conn. However, dependingupon the needs of the specific shielding application, a fabricconstructed of a combination or blend of conductive and nonconductivefibers alternatively may be employed. Examples of fabrics woven,braided, or warp knitted from electrically-conductive fibers, or fromblends of conductive and non-conductive fibers, are described inGladfelter, U.S. Pat. No. 4,684,762, and in Buonanno, U.S. Pat. No.4,857,668.

[0032] Returning to FIGS. 1 and 2, coating member 14 preferably isformed from a curable layer of a fluent, flame retardant resin or othercomposition which is wet coated onto the second side 18 of fabric member12. As is detailed hereinafter, the viscosity and hydrodynamic pressureof the resin composition are controlled in accordance with the preceptsof the present invention to delimit the penetration of the resin layerto a depth, referenced at “d” in FIG. 2, which is less than thethickness dimension t₁ of the fabric member 12. In this regard, when thelayer is cured to form the flame retardant surface coating member 14 onthe second side 18 of fabric member 12, the first side 16 thereofremains electrically-conductive. In a preferred construction, the layeris coated to a wet thickness of about 10 mils (0.25 mm), and then curedto a dried coating or film thickness, referenced at t₂ in FIG. 2, ofbetween about 2-4 mils (0.05-0.10 mm) at a depth d of about 1-2 mils(0.025-0.05 mm). Ultimately, a total material thickness, referenced at“T,” of between about 6-7 mils (0.15-0.20 mm) and a dried weight pickupof between about 100-150 g/yd² are observed. By “cured” it is meant thatthe resin is polymerized, cross-linked, further cross-linked orpolymerized, vulcanized, hardened, dried, volatilized, or otherwisechemically or physically changed from a liquid or other fluent form intoa solid polymeric or elastomeric phase.

[0033] The flame retardant composition preferably is formulated as anaqueous emulsion of an acrylic latex emulsion which is adjusted to atotal solids of about 60% and a Brookfield viscosity (#5 spindle, 4speed) of between about 40,000-60,000 cps, at a density of about 10 lbsper gallon (1.8 g/cm³). Flame retardancy may be imparted by loading theemulsion with between about 30-50% by weight of one or more conventionalflame retardant additives such as aluminum hydrate, antimony trioxide,phosphate esters, or halogenated compounds such as polybrominateddiphenyl oxides. A preferred formulation is a mixture of about 25% byweight, based on the total weight of the emulsion, of decambromodiphenyloxide and about 15% by weight of one or more antimony compounds. Inoperation, should the acrylic carrier phase be ignited, thedecomposition of the halogenated and metal oxide compounds function tochemically deprive the flame of sufficient oxygen to support combustion.The decomposition of the acrylic phase additionally may lead to thedevelopment of a protective, i.e., thermally-insulative or refractory,outer char layer.

[0034] A preferred flame retardant, acrylic latex emulsion is marketedcommercially by Heveatex Corp., Fall River, Mass., under the designation“4129FR.” The viscosity of the emulsion may be adjusted to between about40,000-60,000 cps using an aqueous acryloid gel or other acrylicthickener. In this regard, the increased viscosity of the emulsioncontributes to delimiting the penetration of the coating layer into thefabric member. However, as this relatively high viscosity may lead toundesirable porosity in the dried film, the emulsion additionally may bemodified to reduce air entrapment and bubble formation in the coatinglayer with up to about 1% by weight of one or more commercialsurfactants such as “Bubble Breaker” by Witco Chemical Corp. (Chicago,Ill.) and “Foam Master Antifoam” by Diamond Shamrock, Inc. (San Antonio,Tex.).

[0035] As aforementioned, EMI shielding material 10 of the presentinvention is particularly adapted for use as a flame retardant,electrically-conductive jacket which is provided over a foam core in anEMI shielding gasket construction such as gasket 50 of FIG. 4. In arepresentative embodiment, gasket 50 includes an elongate, resilientfoam core member, 52, which may be of an indefinite length. Core member52 has an outer circumferential surface, 54, defining thecross-sectional profile of gasket 50 which, for illustrative purposes,is of a generally polygonal, i.e., square or rectangular geometry. Otherplane profiles, such as circular, semi-circular, or elliptical, orcomplex profiles may be substituted, however, depending upon thegeometry of the interface to be sealed. Core member 12 may be of anyradial or diametric extent, but for most applications will have adiametric extent or width of from about 0.25 inch (0.64 cm) to 1 inch(2.54 cm).

[0036] For affording gap-filling capabilities, it is preferred that coremember 52 is provided to be complaint over a wide range of temperatures,and to exhibit good compression-relaxation hysteresis even afterrepeated cyclings or long compressive dwells. Core member 52 thereforemay be formed of a foamed elastomeric thermoplastic such as apolyethylene, polypropylene, polypropylene-EPDM blend, butadiene,styrene-butadiene, nitrile, chlorosulfonate, or a foamed neoprene,urethane, or silicone. Preferred materials of construction include openor closed cell urethanes or blends such as a polyolefin resin/monoolefincopolymer blend, or a neoprene, silicone, or nitrile sponge rubber.

[0037] Core member 52 may be provided as an extruded or molded foamprofile over which shielding material 10 is wrapped as a sheathed, withthe edges of sheathed being overlapped as at 56. In a preferredconstruction, shielding material 10 is bonded to the core member 52 in acontinuous molding process wherein the foam is blown or expanded withinthe shielding material. As may be seen best with reference to themagnified view of FIG. 4 referenced at 60 in FIG. 5, in suchconstruction coating member 14 is disposed adjacent core member 52 as aninterior surface, 62, of shielding member 10, with the uncoated side 16of fabric member 12 being oppositely disposed as anelectrically-conductive exterior surface, 64, of the gasket 50. It willbe appreciated that the coated interior surface 62 blocks the pores 22(FIG. 3) of the fabric member 12 of the fabric to retain the blown foamtherein without penetrate or bleed through to the exterior gasketsurface 64. Depending upon the respective compositions of the foam andcoating, the interior surface 62 may function, moreover, as acompatibilizing or “tie” interlayer which promotes the bonding of thefoam to the fabric. Gasket construction 50 advantageously provides astructure that may be used in very low closure force, i.e. less thanabout 1 lb/inch (0.175 N/mm), applications.

[0038] Referring again to FIG. 4, an adhesive layer, 70, may be appliedalong the lengthwise extent of gasket 50 to the underside of exteriorsurface 64 for the attachment of the gasket to a substrate. Such layer70 preferably is formulated to be of a pressure sensitive adhesive (PSA)variety. As is described in U.S. Pat. No. 4,988,550, suitable PSA's forEMI shielding applications include formulations based on silicones,neoprene, styrene butadiene copolymers, acrylics, acrylates, polyvinylethers, polyvinyl acetate copolymers, polyisobutylenes, and mixtures,blends, and copolymers thereof. Acrylic-based formulations, however,generally are considered to be preferred for the EMI applications of thetype herein involved. Although PSA's are preferred for adhesive layer70, other adhesives such as epoxies and urethanes may be substitutedand, accordingly, are to be considered within the scope of the presentinvention. Heat-fusible adhesives such a hot-melts and thermoplasticfilms additionally may find applicability.

[0039] Inasmuch as the bulk conductivity of gasket 50 is determinedsubstantially through its surface contact with the substrate, anelectrically-conductive PSA may be preferred to ensure optimal EMIshielding performance. Such adhesives conventionally are formulated ascontaining about 1-25% by weight of a conductive filler to yield avolume resistivity of from about 0.01-0.001 Ω-cm. The filler may beincorporated in the form of particles, fibers, flakes, microspheres, ormicroballoons, and may range in size of from about 1-100 microns.Typically filler materials include inherently conductive material suchas metals, carbon, and graphite, or nonconductive materials such asplastic or glass having a plating of a conductive material such as anoble metal or the like. In this regard, the means by which the adhesiveis rendered electrically conductive is not considered to be a criticalaspect of the present invention, such that any means achieving thedesired conductivity and adhesion are to be considered suitable.

[0040] For protecting the outer portion of adhesive layer 70 which isexposed on the exterior surface of the gasket, a release sheets, shownat 72, may be provided as removably attached to the exposed adhesive. Asis common in the adhesive art, release sheet 72 may be provided as stripof a waxed, siliconized, or other coated paper or plastic sheet or thelike having a relatively low surface energy so as to be removablewithout appreciable lifting of the adhesive from the exterior surface64.

[0041] In the production of commercial quantities of the EMI shieldingmaterial 10 of the present invention, the viscosity adjusted andotherwise modified acrylic latex emulsion or other resin composition maybe coated and cured on one side the fabric member 12 by a direct wetprocess such as knife over roll or slot die. With whatever process isemployed, the hydrodynamic pressure of the resin composition iscontrolled in accordance with the precepts of the present invention todelimit the penetration of the resin layer to a depth which is less thanthe thickness dimension of the fabric member. For example, and withreference to FIG. 6 wherein the head of a representative gravity-fedknife over roll coater is shown somewhat schematically at 100, porous,i.e., permeable, fabric member 12 is conveyed from a feed roll or thelike (not shown) over a nip roller, 102, which rotates in the directionreferenced by arrow 104. With the first side 16 of fabric member 12supported on roller 102, the fabric second side 18 is passed beneath theopening, referenced at 106, of a coating trough, 108. Trough 108 isdefined by a front plate, 110, a back plate, 112, and a pair of sideplates (not shown).

[0042] The emulsion or other fluent resin composition, referenced at114, is pumped or otherwise transported into trough 108 which is filledto a fluid level, referenced at h. For a given fluid density, this levelh is controlled such that the hydrodynamic pressure at the fabric-liquidinterface is maintained within preset limits. For example, with a fluiddensity of about 10 pounds per gallon (1.8 g/cm³), and a fabric having aporosity of about 1000-2000 openings per inch with a mean average poresize of between about 0.5-2 mils (12.5-50 μm), the fluid level H iscontrolled at about 4 inches (10 cm) to yield a hydrodynamic pressure ofabout 0.05 psi (0.35 kPa) at the fabric-liquid interface. For othercoating processes, the hydrodynamic fluid pressure may be controlled,for example, by a pumping pressure or the like.

[0043] In the illustrative knife-over-roll coating process, the loweredge, 120, of front plate 110 defines a knife surface which is shimmedor otherwise spaced-apart a predetermined distance from the second side18 of fabric member 12. Such spacing provides a clearance or gap,referenced at “g,” of typically about 10 mils (0.25 mm), but which isadjustable to regulate the thickness of the liquid coating layer, 122,being applied to the fabric member. From roller 104, the coated fabricmember 12 may be conveyed via a take-up roller arrangement (not shown)through a in-line oven or the like to dry or flash the water or otherdiluent in the liquid coating layer 122, or to otherwise cure the liquidcoating layer 122 in developing an adherent, tack-free, film or otherlayer of coating member 14 (FIG. 1) on the single side 18 of fabricmember 12.

[0044] The Example to follow, wherein all percentages and proportionsare by weight unless otherwise expressly indicated, is illustrative ofthe practicing of the invention herein involved, but should not beconstrued in any limiting sense.

EXAMPLE

[0045] Representative EMI shielding materials according to the presentinvention were constructed for characterization. In this regard, amaster batch of a flame retardant coating composition was compoundedusing an acrylic latex emulsion (Heveatex “4129FR”). The viscosity ofthe emulsion was adjusted to a Brookfield viscosity (#4 spindle, 40speed) of about 60,000 cps with about 5 wt % of an acryloid thickener(Acrysol™ GS, Monsanto Co., St. Louis, Mo.). The modified emulsion had atotal solids content of about 60% by weight, a density of about 10pounds per gallon (1.8 g/cm³), and a pH of between about 7.5 and 9.5.

[0046] The emulsion was applied using a knife over roll coater (JETZONEModel 7319, Wolverine Corp., Merrimac, Mass.) to one side of asilver-plated nylon fabric (Swift “31EN RIPSTOP”) having a thickness ofabout 4 mils (0.1 mm). With the fluid level in the coating trough of thecoater maintained at about 4 inch (10 cm), the emulsion was delivered tothe surface of the cloth at a hydrodynamic pressure of about 0.05 psi(0.35 kPa). The coating knife was shimmed to a 10 mil (0.25 mm) gapabove the fabric to yield a wet coating draw down thickness of about 10mils. Following an oven curing at 100-125° C. for 5 minutes, a driedcoating or film thickness of about 2.5 mils (0.635 mm) was obtained witha weight pickup of about 130-145 g/yd² and a total material thickness ofbetween about 6-7 mils (0.15-0.18 mm). An inspection of the coatedfabric cloth revealed a coating penetration depth of about 1-2 mils(0.02-0.05 mm) providing acceptable mechanical retention and/or adhesionof the coating onto the fabric surface. The opposite side of the fabric,however, was observed to be substantially coating free, and to retain asurface resistivity of about 0.1 Ω/sq for unaffected EMI shieldingeffectiveness.

[0047] Fabric samples similarly coated in the manner described weresubjected to an in-house vertical flame test. No burning was observed atdried film thickness of 2, 3, or 4 mils (0.05, 0.08, 0.10 mm).Accordingly, a reasonable operating window of film thickness wassuggested for production runs.

[0048] Samples also were provided, as jacketed over a polyurethane foamcore in an EMI shielding gasket construction, for flame testing byUnderwriters Laboratories, Inc., Melville, N.Y. A flame class rating ofV-0 under UL94 was assigned at a minimum thickness of 1.0 mm. The gasketconstruction therefore was found to be compliant with the applicable ULrequirements, and was approved to bear the “UL” certification mark.

[0049] The foregoing results confirm that, the EMI shielding material ofthe present invention affords UL94 V-0 protection when used as ajacketing in a fabric-over-foam gasket construction. Unexpectedly, itwas found that a relatively porous or permeable fabric may be wet coatedon one side with a relatively thin, i.e., 2-4 mil (0.05-0.10 mm),coating layer of a flame retardant composition without compromising theelectrical surface conductivity of the other side. Such a thin coatinglayer, while being sufficient to provide UL94 V-0 protection in aconventional fabric-over-foam gasket construction, nonetheless maintainsthe drapability the fabric and thereby facilitates the fabrication ofUL94 V-0 compliant gaskets having complex profiles or narrowcross-sections down to about 1 mm.

[0050] As it is anticipated that certain changes may be made in thepresent invention without departing from the precepts herein involved,it is intended that all matter contained in the foregoing descriptionshall be interpreted as illustrative and not in a limiting sense. Allreferences cited herein are expressly incorporated by reference.

What is claimed is:
 1. A flame retardant, electromagnetic interference(EMI) shielding gasket comprising: a resilient core member extendinglengthwise along a central longitudinal axis and having an outer surfaceextending circumferentially about said longitudinal axis, said coremember being formed of a foamed elastomeric material; anelectrically-conductive fabric member surrounding the outer surface ofsaid core member, said fabric member having an interior surface disposedfacing the outer surface of said core member and an oppositely-facingexterior surface; and a flame retardant layer coating at least a portionof the interior surface of said fabric member, said flame retardantlayer being effective to afford said gasket a flame class rating of V-0under Underwriter's Laboratories (UL) Standard No.
 94. 2. The gasket ofclaim 1 wherein said flame retardant layer has a thickness of betweenabout 2-4 mils (0.05-0.10 mm).
 3. The gasket of claim 1 wherein saidflame retardant layer of is formed as a cured film of a flame retardantacrylic latex emulsion.
 4. The gasket of claim 1 wherein said fabricmember is a metal-plated cloth.
 5. The gasket of claim 4 wherein saidcloth comprises fibers selected from the group consisting of cotton,wool, silk, cellulose, polyester, polyamide, nylon, and combinationsthereof, and said metal is selected from the group consisting of copper,nickel, silver, nickel-plated-silver, aluminum, tin, and combinationsthereof.
 6. The gasket of claim 1 wherein said foamed elastomericmaterial is selected from the group consisting of polyethylenes,polypropylenes, polypropylene-EPDM blends, butadienes,styrene-butadienes, nitriles, chlorosulfonates, neoprenes, urethanes,silicones, and polyolefin resin/monoolefin copolymer blends, andcombinations thereof.
 7. The gasket of claim 1 wherein said fabricmember has a thickness of between about 2-4 mils (0.05-0.10 mm).